white balance is what you do to the camera to set it for a particular situation. In other words, it sets the camera up for shooting in different light conditions. The more expensive and professional cameras will have this.

I'll get back to you with the gain.

Do you have any ideas for a low budget spy / espionage film that i am planning on writing... My budget? = ?0.00

Gain is just a way of making the camera more sensitive to light, similar to pushing film. It has side effects though. It can result in a noisier image with washed out blacks and blown-out whites. It is usually not a preferred method of getting a good exposure.